The present invention relates to autoregulating electric heaters and more particularly, to electro-magnetic autoregulating electric heaters that are operable with low frequency energy sources but which do not radiate harmful electro-magnetic fields.
In the U.S. Pat. No. 4,256,945 of Carter and Krumme, there is described an autoregulating electric heater having a laminated structure; one lamina of which has high magnetic permeability and high resistance and another lamina of which is non-magnetic and has a low resistance (such as copper) in electrical contact, and therefore, thermal contact with the first lamina. This structure is adapted to be connected across a constant current, a.c. source such that the layers are in a sense in parallel across the source.
Due to skin effect, the current is initially confined to the high magnetic permeability, high resistance layer so that P - KR.sub.1 where P is power, K is I.sup.2 which is constant, and R is the effective resistance of the permeable material. The dissipation of power heats the layer until it approaches its Curie temperature. The permeability of the lamina decreases towards the level of the second layer, copper for instance, at about its Curie temperature. The current is no longer confined to the high resistivity first lamina by the magnetic properties of the first lamina, and spreads into the copper layer; the effective resistance of the structure drops materially. the power consumed, P=KR.sub.2 where R.sub.2 &lt;&lt;R.sub.1, is greatly reduced, and the heating effect is reduced to a level that maintains the device at or near Curie temperature. The device thus thermally autoregulates over a narrow temperature range about the Curie temperature.
The current source employed in the aforesaid patent is typically a high frequency source, for instance, 8 to 20 MHz to insure that the current is confined to the thin, high resistance, magnetic layer until the Curie temperature of the magnetic material is attained. Specifically, the maximum regulation is achieved when the thickness of the magnetic layer is of the order of one skin depth at the frequency of operation. Under these circumstances, the maximum change in effective resistance of the structure is achieved at or about the Curie temperature. This fact can be demonstrated by references to the equation for skin depth in a monolithic, i.e., non-laminar magnetic structure: ##EQU1## where .rho. is the resistivity of the the material in ohm-cms, .mu. is magnetic permeability and .function. is frequency of the current. The field falls off in accordance with e.sup.-x where X=thickness/skin depth. Accordingly, in a monolithic structure, by calculation, 63.2% of the current is confined to one skin depth in the high mu material. In the region of the Curie temperature, where .mu. 1, the current spreads into a region ##EQU2## If mu was originally equal to 400, the skin depth in the region at the Curie temperature increases by the square root of 400; i.e., the skin depth in the monolithic structure is now 20 times greater than with .mu.=400.
The same type of reasoning concerning the skin effect may be applied to the two layer laminar structure in the aforesaid patent. Below the Curie temperature, the majority of the current flows in the magnetic layer when the thickness of this layer is nominally one skin depth of the material below the Curie temperature. In the region of the Curie temperature, the majority of the current now flows in the copper and the resistance drops dramatically. If the thickness of this high mu material were greater than two skin depths, for instance, with .mu.=400, the percentage change of current flowing in the high conductivity copper would be less and the resistivity change would not be as dramatic. Similarly, if the thickness of the high mu material were materially less than one skin depth, the percentage of current flowing in the high resistivity material at a temperature less than the Curie temperature would be less so that the change of resistance at the Curie temperature would again not be as dramatic. The region of 1.0 to perhaps 1.8 skin depths of high mu material is preferred.
An exact relationship for the two layer case is quite complex. The basic mathematical formulas for surface impedance from which expressions can be obtained for the ratio of the maximum resistance, R.sub.max, below the Curie temperature, to the minimum resistance, R.sub.min, above the Curie temperature, are given in Section 5.19, pp. 298-303 of the standard reference, "Fields and Waves in Communications Electronics," 3rd Edition, by S. Ramo, J. R. Whinnery, and T. VanDuzer, published by John Wiley and Sons, New York, 1965. Although the theory described in the above reference is precise only for the case of flat layers, it is still accurate enough for all practical applications in which the skin depth is substantially less than the radius of curvature.
The above facts are clearly demonstrated by the curves A and B of the graphs of FIG. 1 hereof which are based on the two-layer theory in the above reference. The curves are continuous with the parts lying to the left and right of the vertical dashed line being read against the left and right scales, respectively. These curves are plots of autoregulation power ratio as a function of frequency of the current applied to the patented devices. The maximum autoregulation power ratio is achieved at 0.6 MHz for a material having a mu of 600 .rho..sub.1 =75.times.10.sup.-6 and a thickness of 1.5.times.10.sup.-3 inch as illustrated in Curve B. In this case, one skin depth is 0.9.times.10.sub.-3 inch with a peak ratio of 162 occurring at frequency of 600 KHz. As indicated above, the thickness of the high mu layer is 1.5.times.10.sup.-3 inch. Thus, the theoretical optimum thickness is 1.67 times one skin depth thickness below the Curie temperature.
The curve A is for a body having a first lamina of a thickness of 0.5.times.10.sup.-3 inch. It is noted that the peak ratio of 160 is attained at 6 MHz, at which frequency the skin depth in the magnetic material is 0.29.times.10.sup.-3 inches.
Difficulty may arise in such devices when the Curie temperature is achieved due to spread of the current into adjacent regions outside of the device, particularly if the device is located close to sensitive electrical components.
In copending patent application Ser. No. 243,777, filed Mar. 16, 1981, a continuation-in-part application of the application from which the aforesaid patent matured, there is described a mechanism for preventing the high frequency field generated in the heated device from radiating into the regions adjacent the device. This effect is accomplished by insuring that the copper or other material of high conductivity is sufficiently thick, several skin depths at the frequency of the source, to prevent such radiation and electrical field activity. This feature is important in many applications of the device such as a soldering iron where electromagnetic fields may induce relatively large currents in sensitive circuit components which may destroy such components.
As indicated above, the magnetic field in a simple, single layer, i.e., monolithic structure, falls off as e.sup.-x so that at three skin depths, the field is 4.9% of maximum, at five skin depths, it is 0.6%, and at ten skin depths, the field is 0.004% of maximum. For many uses, thickness of three skin depths are satisfactory although ten or more may be required with some highly sensitive devices in the vicinity of large heating currents.
The devices of the patent and application are operative for their intended purposes when connected to a suitable supply, but a drawback is the cost of the high frequency power supply. Where only a very low field may be permitted to radiate from the device, the frequency of the source is preferably maintained quite high, for instance, in the megahertz region, to be able to employ copper or other non-magnetic material having reasonable thicknesses.